High-energy O3-NaCa[NiMn]O cathodes for long-life sodium-ion batteries

To facilitate the practical realization of sodium-ion batteries, the energy density, determined by the output operating voltage and/or capacity, needs to be improved to the level of commercial Li-ion batteries. Herein, O3-type Na 0.98 Ca 0.01 [Ni 0.5 Mn 0.5 ]O 2 is synthesized by incorporating Ca 2+ into the NaO 6 octahedron of Na[Ni 0.5 Mn 0.5 ]O 2 and its potential use as a cathode material for high energy density SIBs is demonstrated. The ionic radius of calcium ( 1.00 Å) is similar to that of sodium ( 1.02 Å); hence, it is energetically favorable for calcium to occupy sites in the sodium layers. Within a wide operating voltage range of 2.0-4.3 V, O3-type Na 0.98 Ca 0.01 [Ni 0.5 Mn 0.5 ]O 2 exhibits a reversible O3-P3-O3 phase transition with small volume changes compared to Ca-free Na[Ni 0.5 Mn 0.5 ]O 2 because of the strong interaction between Ca 2+ and O 2− and delivers a high reversible capacity of 209 mA h g −1 at 15 mA g −1 with improved cycling stability. Moreover, Ca substitution improves the practically useful aspects such as thermal and air stability. A prototype pouch full cell with a hard carbon anode shows an excellent capacity retention of 67% over 300 cycles. Thus, this study provides an efficient and simple method to boost the performance and applicability of layered oxide cathode materials for practical applications. Substituting Ca for Na sites in the O3-Na[Ni 0.5 Mn 0.5 ]O 2 cathode provides outstanding stability at high voltage where Na-ions are fully extracted.